This application is directed to a system for the cathodic protection of reinforcing members referred to as xe2x80x9crebarsxe2x80x9d in conventionally reinforced concrete structures. Such rebars are produced from mild steel (also referred to as xe2x80x9cblack steelxe2x80x9d) which has less than 1% carbon and less than 2% of alloying elements, combined. More particularly the invention teaches a method of providing cathodic protection which is immediately commenced on newly embedded rebars in reinforced and/or prestressed concrete structures, that is, structures such as bridges, buildings including power stations, marine structures such as docks, and roadways which are yet to be built.
In applications where the cost of corrosion-protected rebars is justified, they have been coated with a synthetic resinous layer, typically an epoxide resin, which serves as a barrier against any liquid, thus denying formation of an electrochemical cell on the surface of the rebar. Such protection is referred to as xe2x80x9cbarrier protectionxe2x80x9d and is sometimes also obtained by painting rebars with a wide array of paints. Alternatively, rebars have been galvanically protected by being hot-dipped in zinc. Another alternative is to provide a rebar with both galvanic protection, and barrier protection. For example, some paints contain a high concentration of conductive metal such as zinc powder, or, metal salts such as zinc chromate.
Galvanized and aluminized steel products are commonplace as is the use of aluminum as an anodic metal. It is recognized that a thin aluminum film less than 0.2 mm thick, by itself, has limited protective function as a sacrificial anode because there is insufficient aluminum metal to be sacrificed over a long period of time in the range from 20 to 50 years. It is also recognized that a thicker coating of aluminum in the range greater than 0.2 mm thick up to about 1 mm thick, will provide effective protection as a sacrificial anode provided the aluminum itself is not destroyed by corrosive forces of its environment. Such corrosive forces exist in freshly poured concrete which has a pH above 9, up to about pH 13, which pH remains above 9 for several years while the concrete is curing, typically up to about 5 years, after which carbonation of the concrete, and acidification due to sulfur trioxide, acidic water and other factors, start to lower the pH of the concrete.
U.S. Pat. No. 5,100,738 to Graf teaches coating a rebar of a xe2x80x9csteel alloy customary in reinforcing steelsxe2x80x9d (col 1, lines 58-59) immediately after rolling, with a layer of aluminum or aluminum alloy (together referred to as the xe2x80x9cAl-layerxe2x80x9d), then coating the once-coated rebar with a layer of a synthetic resin (xe2x80x9cfirst layerxe2x80x9d). The stated purpose of the Al-layer is that it xe2x80x9censures reliable corrosion protection, in particular even when cracks appear in the first layer when in use, i.e. in particular upon bending of the reinforcing steel. In such cracks the second layer of aluminum or of aluminum alloy is exposed so that, until the concrete of a concrete structural part in which the reinforcing steel is imbedded sets, this layer then reacts with the free lime of the concrete of the cement with the assistance of oxygen to form a calcium aluminate, which ensures particularly solid and tight fusion with the reinforcing steel, so that no cracks, etc. into which moisture can penetrate occur or remain between the reinforcing steel and concrete. The first layer protects the second layer against external stresses of a chemical and/or mechanical nature.xe2x80x9d (see col 1, lines 21-36). This statement of how the Graf reinforcing member functions is reiterated at col 2, lines 27-48).
Since the function of the Al-layer is to provide the metal reactant for the free lime so as to form the calcium aluminate, there is no need for a thicker layer of Al metal than is required for the chemical reaction. Therefore Graf specifies that the Al-layer xe2x80x9cis under 200 xcexcm (micrometers)xe2x80x9d, preferably xe2x80x9cin the order of magnitude of about 20 to 25 xcexcmxe2x80x9d. Upon reaction the aluminum provides the calcium aluminate which xe2x80x9censures particularly solid and tight fusion with the reinforcing steelxe2x80x9d and concrete.
However, the desired reaction to form the calcium aluminate is not the only function of the Al coating because Graf states the Al-layer xe2x80x9ccontains zinc, while the percentage of aluminum is greater than 50% and, preferably, between about 55% and 70%.xe2x80x9d and, that xe2x80x9cthe percentage of zinc is smaller than 50% and, preferably, between about 28% and 43%.xe2x80x9d (see col 1, lines 44-49).
There is no teaching in Graf as to how the desired thin Al-layer is applied. However it is known that a layer of Al less than 200 xcexcm thick conventionally applied on a rebar cannot be non-porous, and recognizing this, Graf also uses his Al-layer of Al-alloy to function as a sacrificial anode.
There is no indication as to whether the duty of the thin layer as a sacrificial anode is completed before the chemical reaction forming the calcium aluminate occurs because it is evident that if the calcium aluminate were to be formed first, there would be no protection from a sacrificial anode. Therefore one skilled in the art will appreciate that, depending upon the thickness of the metal layer, it will provide at least a measure of cathodic protection of the rebar, by virtue of the metal layer functioning as a sacrificial anode. Thus, with the Graf rebar protected with a coating of aluminum functioning both as a reactant and a sacrificial anode, it is clear that there is no reason to use such an anodically protected rebar as a cathode.
If there was no synthetic resinous layer overlying the very thin Al-layer on the rebars, then upon the rebars being embedded in freshly poured concrete, their entire surface would be transformed into a very thin calcium aluminate surface which presumably would not be expected to corrode. Since the calcium aluminate provides barrier protection, and there is no suggestion or reason to believe that the calcium aluminate layer provides any galvanic protection, it is evident that Graf did not believe such rebars could be used with an impressed current. Further, since Graf deliberately coated the Al-layer with an epoxy resin (stated only in claim 8) which is known to be electrically non-conductive, it would not be reasonable to use such a rebar with an impressed cathodic current of practical magnitude.
Still further, it is known that where an insulating layer of resin is provided on a metal surface which is then cathodically protected with an impressed current, and a break, crack or fissure exists in the resin which exposes the metal surface within the fissure, the exposed metal surface is protected, but the metal surface proximately surrounding the fissure becomes corroded causing the resin immediately above the corroding surface to be lifted from the metal surface. This phenomenon is more fully explained in a reference text titled xe2x80x9cHandbuch des Kathodischen Korrosionsshutzes, 1980 (164-173)xe2x80x9d which in relevant part states xe2x80x9cSpecific damage to steels results when a coating of non-ferrous metal (such as aluminum) is overlaid with a coating of resin and used in a cathodically protected system in which there is a break in the resin coating and the environment penetrates the break. Such damage is referred to as cathodic detachment. The generation of hydrogen by electrochemical reaction leads to separation of the resin coating and destruction of the metal with a high rate of corrosion.xe2x80x9d
Rather than forming a calcium aluminate and relying upon that for protection, this invention relies upon the discovery that a continuous and uninterrupted essentially non-porous thin aluminum oxide (Al-oxide) layer, or, hydrated Al-oxide (HAl-oxide) layer, less than 100 xcexcm thick, typically in the range from 5 xcexcm to 75 xcexcm thick, on the surface of essentially pure aluminum coated on a rebar survives in freshly poured concrete having a pH above 9 and up to about 13, long enough to protect the Al metal until the concrete sets. Hereafter the term xe2x80x9ccombined Al-oxide layerxe2x80x9d refers to a thin coating of aluminum oxide, or hydrated aluminum oxide, or both. This combined Al-oxide layer is corrosion resistant until destroyed.
Further, the combined Al-oxide layer fails to operate effectively to limit an impressed current sufficient to counter the potential at the cathode. The invention relies upon maintaining the combined Al-oxide coating for an arbitrarily long time despite the changing pH of the concrete environment of the rebar. The presence of the Al-oxide layer not only provides barrier protection but also unexpectedly lowers the current density (mA/m2) required to provide effective cathodic protection relative to that required to protect virgin rebars which are not coated. The novel Al-coated rebars, without a coating of resin, are nevertheless doubly protected with two layers, (i) a first layer of essentially pure Al in contact with the rebar, and (ii) a second layer of alumina (Al2O3) overlying the layer of Al. Hereafter the term xe2x80x9cAl-coatedxe2x80x9d refers to such a doubly protected rebar. Such Al-coated rebars have been found to be sufficiently conductive to be galvanically protected, preferably with magnesium.
Because an Al-oxide film forms essentially instantaneously on pure Al, and Al-coated rebars are embedded initially in an aqueous concrete environment, what is of interest are the Pourbaix diagrams for aluminum with an Al-oxide layer, and aluminum with a HAl-oxide layer. The Al-oxide layer shows immunity or passive behavior in the pH range from about 5 to 9; the HAl-oxide film shows immunity or passive behavior in the pH range from about 3 to 8.5 (see Corrosion Data, Aluminum and Aluminum Alloys, pg 16).
In a galvanic circuit, the metal to be protected becomes the cathode to which the anode is connected. For example, relative to the standard potential (in volts) at 25xc2x0 C. of Hydrogen Reference Electrode (HRE)=0 V, of iron (Fe) is xe2x88x920.440 V; that for zinc (Zn) is xe2x88x920.763 V; that for Al is xe2x88x921.66 V; and that for magnesium (Mg) is xe2x88x922.37 V. The standard potential for Fe is given for the electrode reaction Fe2++2exe2x88x92=Fe; the potential for Al is given for the electrode reaction Al3++3exe2x88x92=Al; the potential for Zn is given for the electrode reaction Zn2++2exe2x88x92=Zn; and the potential for Mg is given for the electrode reaction Mg2++2exe2x88x92=Mg. In existing structures, the metals have corrosion potentials which will vary depending upon the environment. In a typical natural environment, the corrosion potential for Fe is in the range from xe2x88x920.35 to xe2x88x920.45 V, on average xe2x88x920.4 V; for Zn is in the range from xe2x88x920.70 to xe2x88x920.80 V, on average xe2x88x920.75 V; for Al is in the range from xe2x88x920.50 to xe2x88x920.60 V, on average xe2x88x920.55 V; for Mg is in the range from xe2x88x921.20 to xe2x88x921.40 V, on average xe2x88x921.30 V. Therefore, as is well known, Al does not behave as would be expected by virtue of its position in the EMF series.
Accordingly, aluminum or aluminum-rich alloy rods, or magnesium and magnesium-rich alloy rods, zinc and zinc-rich alloys were used as sacrificial anodes proximately disposed or embedded within the structure in galvanic connection with the steel rebars; or zinc-coated rebars were used; in either case, the required mass of the anode is the amount of metal which goes into solution over time, this amount of metal being the amount of electricity flowing through the galvanic circuit and the time over which the metal is consumed (Faraday""s law). Since protection is sought over an extended time, and the rate of consumption of the anode is typically quite high once corrosion commences, the required mass of sacrificial anode for the long period, say 100 years, is high. Moreover, periodic replacement of anodes to provide continuous protection is inconvenient at best and often impractical. Therefore use of such sacrificial anodes has been largely discontinued in favor of using an external power supply to provide an impressed cathodic current to the corrodible metal. By controlling the impressed current the service life of the structure is not limited by corrosion of its steel reinforcement.
To avoid confusion, it should be noted that in galvanic cells, the cathode is the positive pole and the anode is the negative pole. The electrode at which chemical reduction occurs (or + electricity enters the electrode from the electrolyte) is called the cathode (e.g. H+xe2x86x921/2H2xe2x88x92exe2x88x92); and the electrode at which chemical oxidation occurs (or + electricity leaves the electrode and enters the electrolyte) is called the anode (e.g. Znxe2x86x92Zn2++2exe2x88x92). However, when current is impressed on a cell from a generator or an external battery, reduction occurs at the electrode connected to the negative pole of the external current source, and this electrode is therefore the cathode. Thus the cathode is the electrode at which current enters from the electrolyte, and the anode is the electrode at which current leaves to return to the electrolyte. Cations migrate towards the cathode when electricity flows through the cell and are positively charged; anions are negatively charged.
In cathodic protection, an impressed current is caused to flow through the anode into the electrolyte and then to the rebars in the structure. Such protection with the uncoated steel rebars as the cathode, as conventionally practiced, is expensive, requiring a much higher current density to obtain a satisfactorily low level of corrosion than that required to obtain the same corrosion protection with rebars coated with Al.
The real benefit of electrochemical protection is that one can obtain equivalent protection at much lower current density. This protection occurs when the electrochemical nature of the cladding comes into play. Further corrosion spreads laterally confining itself to the aluminum oxide and/or hydrated oxide cladding rather than penetrating into the steel core of the cathode. The rate of attack is affected by the relative size of the anode and the pH of the concrete environment; a small anode area in contact with a large cathode area will result in a rapid and severe attack. Because the degree of ionization of the cladding is so low, the rate of attack is low.
Despite numerous teachings as to how rebars may be protected against corrosion in concrete, current construction routinely uses virgin rebars which have been cut to length in a rolling mill and which have been oxidized in the atmosphere in which they were stored. Since the oxided (ferrous and ferric oxides) coating on a rebar provides it with a substantial level of protection against the alkaline environment in freshly poured and cured concrete, there has been little incentive to protect rebars any further.
It is well known that aluminum and aluminum alloys may be cathodically protected with a sacrificial anode of magnesium or alloy lower in the electromotive series (that is, having a lower or more negative potential) than aluminum, but it is far more practical to provide protection with an impressed cathodic current. In an impressed current circuit, the article to be protected is the cathode, and the anode may be consumable but preferably is graphite or other non-consumed metal or alloy. The cathode and anode in a concrete environment provides salts dissolved in water as the electrolyte medium.
The concrete environment which is continually changing differentiates it from those for which numerous other cathodic protection systems are provided. Such other systems are provided for the hulls of boats and other large aluminum articles. Such articles differ greatly from rebars in that they have all relatively thick cross-sections of aluminum or aluminum alloys, typically at least 3 mm thick, and they are not in a concrete environment. Such thickness provides a large measure of latitude with respect to control of the impressed current because the pH of the immediate surroundings of the cathode and anode, for example sea water, changes within a relatively narrow range of from about pH 8 to 10.
U.S. Pat. No. 4,510,030 to Miyashita et al recognized the problem of corrosion of aluminum xe2x80x9chaving an anode oxide coating or a film of paint applied to the surface thereof or bare aluminum materials, immersed in water, against pitting or grain boundary corrosion by the application of the aforementioned sacrificial anode or cathodic protection method.xe2x80x9d (see col 2 lines 1-5). They teach that xe2x80x9caluminum materialxe2x80x9d will remain stable in water for a long time without undergoing substantial corrosion if the natural potential of the aluminum xe2x80x9cis maintained in the narrow range from about 03 V to about 0.4 V below the pitting potential up to the pitting potential, . . . xe2x80x9d (see col 2, lines 17-19). However, they teach that xe2x80x9cwhen the voltage of the external power source is controlled so as to maintain the cathode potential at the portion in the vicinity of the opposite electrode of the aluminum material in a proper range, the potential at the portion remote from the opposite electrode is insufficiently repressed. On the other hand, when it is contemplated to repress sufficiently the potential at the portion remote from the opposite electrode of the aluminum material, the potential at the portion in the vicinity of the opposite electrode is excessively repressed. Such excessive repression of the potential tends to cause dissolution, i.e. alkali corrosion, of the aluminum material. As described above, when the conventional sacrificial anode method or cathodic protection method with use of the external power source is relied on, it is difficult to effect control of the cathode potential of the entire volume of the aluminum material so that the potential may remain in the stable range.xe2x80x9d (see col 2, line 61 to col 3, line 10).
Though Miyashita et al do not refer to the pH range which they wish to maintain, or to the range of current density (mA/m2) required, it is clear that their system is directed to a sea water environment where the pH is about 9, and there is no suggestion that they can cope with a pH which is typically initially about pH 13. It is equally clear that they maintain a cathode potential in the range from xe2x88x92700 mV to about xe2x88x921300 mV relative to a calomel electrode (see FIG. 2 of the ""030 patent). Within this range a stable potential is maintained in the range from xe2x88x92700 mV to about xe2x88x921000 mV such that the Al is stable. The circuits shown in FIGS. 1a and 1b (of ""030) are short circuited when the potential reaches xe2x88x92700 mV which returns the potential to about xe2x88x921300 mV. Because they can measure the potential at the protected aluminum surface itself they measure potential as it gradually changes until it approaches the corrosion potential of xe2x88x92700 mV when the current is switched on for a short time. They can never measure the changing potential as corrosion potential changes due to changing environmental conditions while the impressed current is on, and therefore cannot adjust the current as required. They can only short circuit. This deficiency is addressed in the present invention by using a corrosion potential sensing member connected in a circuit separate from the circuit which provides the impressed current for the rebars to be protected. In an environment of freshly poured concrete, the pH is initially in the range from about 12-14; upon commencing to cure the pH remains above pH 9 for about 50 years, after which the pH gradually decreases due to acidification of the concrete, into the range from about pH 5 to pH 9. In concrete with such high alkalinity any additional alkalinity due to a relatively low impressed current proves to be surprisingly insubstantial.
It is a general object of this invention to minimize, if not negate the damage caused by corrosion products of mild steel rebars, which products occupy a larger volume than the metal consumed; not only are the rebars weakened but also the concrete, which cracks and spalls.
It has been discovered that steel rebars coated with an essentially non-porous thin layer of essentially pure aluminum in the range greater than 250 xcexcm but less than about 2 mm thick, preferably in the range from about 250 xcexcm to 1 mm thick, non-removably integrated onto the surface of the rebars, and allowed to form a layer of substantially non-conductive aluminum oxide and/or hydrated aluminum oxide on the surface, function effectively as a cathode. Such rebars with a combined Al-oxide layer may be used with (a) an impressed current and an insoluble anode, or (b) a sacrificial soluble anode; in each case the service life of the protected structure is increased for an arbitrary and indefinitely long period. It is critical that the aluminum coating be of essentially pure aluminum which contains less than 2% of other metals and silicon combined, and that the pH in a zone immediately surrounding the rebar and its Al-oxide layer be maintained in a range in which the rate of corrosion is minimal, typically from about pH 9 to pH 6 though the initial pH of freshly poured concrete is about 13 and will typically decrease to pH 9 or lower after the concrete is exposed to an acidic environment over a period greater than 50 years. By xe2x80x9cimmediately surroundingxe2x80x9d is meant a zone within a radius of 10 mm from the surface of an Al-coated rebar. By xe2x80x9cminimalxe2x80x9d is meant less than 20 xcexcm/yr and preferably less than 10 xcexcm/yr.
It is therefore a general object of this invention to provide a method for protecting steel components including reinforcing members such as rebars in re-inforced and prestressed concrete structures, by coating the rebars with the aforesaid thin essentially pure aluminum coating and allowing them to develop an Al-oxide coating in the range from about 0.1 xcexcm to 100 xcexcm thick before pouring the concrete around them so that the oxide surface is in direct contact with the concrete and is free from an additional layer of synthetic resinous material; and, electrically connecting the essentially non-conductive oxide as cathode in a circuit in which either an insoluble or soluble anode may be used to provide an impressed cathodic current, either anode being used on the surface of the structure, or, in close proximity with the structure, or, within it. Irrespective of the choice of effective positioning of the anodes, the anodes are an essential component of the protected structure and deemed to be essentially integral therewith.
It has unexpectedly been found that the cathodic Al-coating used herein is from 5 to 10 times thinner than a prior art coating which would have been used galvanically to afford the same protection against corrosion of steel rebars in concrete; moreover, using the aforesaid Al-coating reduces the amount of current required for the same degree of cathodic protection afforded by conventional cathodic protection of uncoated rebars, by a factor in the range from about 10 to 20, typically requiring a current density in the range from about 20 to 40 mA/m2; the Al-oxide coating affords sufficient conductivity at a pH in the range from about 6 to 9 to have a surprisingly great effect on the cost of operation for protection allowing corrosion of 10 xcexcm/yr compared to the cost of protection of coated rebars with a sacrificial coating of zinc, offering the same protection. As used hereafter, the term xe2x80x9cAl-coatingxe2x80x9d refers to the coating of essentially pure aluminum which in turn is coated with a xe2x80x9ccombined Al-oxide layerxe2x80x9d.
More particularly, it has been discovered that the Al-coating is surprisingly resistant to alkali corrosion, provided an essentially continuous impressed current is maintained which is in the range from about 150 mV but less than 300 mV, most preferably 200 mV, lower than the corrosion potential of a corrosion potential sensing member of essentially pure Al embedded in concrete in a zone in which the pH is above 9 and up to about pH 13, such member being of the same metal as the coating on the rebars, namely an Al member of arbitrary shape, preferably a plate or rod; the potential of the impressed current required to provide desired protection of the Al-coated rebars in concrete is in the range from about xe2x88x92600 mV (xe2x88x920.6 V) to about xe2x88x921300 m (xe2x88x921.3) relative to a HRE. In the curing or cured reinforced concrete structure, the impressed current represses the cathodic potential of the is rebars to within a predetermined range correlatable with a corrosion potential measured as the corrosion potential sensing member; further, the impressed current maintains a pH in a range from about 6 to about pH 9 in a zone within a radius of about 10 mm from the surface of an Al-coated rebar.